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Islander Transient
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1,881 Posts
Discussion Starter #1
OK, here's an odd one. Does anyone have any idea what the compression should be in a generator engine?
Model: Yamaha YG2800i
Engine size 171 cc

I ran a compression test and it was about 50 PSI. This seems incredibly low as most engines I have tested are closer to 150+ PSI. But I thought that due to the smaller displacement, fact that it's not a high compression engine and it's pull start rather than electric, that the compression might be lower. But 50 seems too low.

Any ideas???
 

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Engine junkie
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142 Posts
The unit most likely has a decompression mechanism used for start up, once fired the decompression mechanism disengages.
To answer the question about compression, it takes a minimum of 90psi for combustion to "correctly" take place.
You may want to do a valve adjustment if not already done, this may clear up some issues too.
 

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Islander Transient
Joined
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1,881 Posts
Discussion Starter #3
The unit most likely has a decompression mechanism used for start up, once fired the decompression mechanism disengages.
To answer the question about compression, it takes a minimum of 90psi for combustion to "correctly" take place.
You may want to do a valve adjustment if not already done, this may clear up some issues too.
Thanks, yes it does have a decompression mechanism and the genny runs fine, although it's louder than I would expect it to be. I do valve adjustments once a year and an oil change twice a season. Just wondering what the correct reading should be. The genny is 6 years old and for the past 3 years I've used it alot so I'm trying to determine if it's time to hone the cylinder and put some new rings in.
 

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Engine junkie
Joined
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142 Posts
If its not smoking, showing signs of oil fouling or excessive carbon build up on the plug then run it.
You can do a leak down test on the cylinder, that will give you better feedback than a compression test.
 
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